This invention relates to laminated ophthalmic spectacles lenses of the type where the distance power of the laminated lens is derived from the combination of front and back wafers each contributing a component of the distance power.
U.S. Pat. No. 3,877,798 proposes the manufacture of bifocal plastic spectacle lenses by gluing together a front and back wafer. The front wafer carries a bifocal part and has an interface curve of approximately 6 diopter as its concave surface and a front curve contributing part of the distance power. The back wafer has an interface curve of approximately 6 diopter as its convex surface and a back curve contributing the remainder of the required distance power. Laminated lens systems of this type use a series of back wafers whose members have a 1/4 diopter difference in curvature of the back curve, if cylinder is ignored.
For non-laminated lenses manufacturers supply a variety of bifocal, trifocal and progressive multifocal semi-finished lenses which have a back curve generated in a grinding and polishing laboratory. Such laboratories use tools which can produce back curves in a series having 1/8 diopter intervals between its members. Such 1/8 diopter intervals allow the laboratory to produce a finished lens falling within the American National standard of .+-.1/16 diopter for the finished lens through power.
Progressive lenses are usually moulded and the moulds are difficult and expensive to produce. It is desirable that a range of laminate lens front wafers be produced using the same moulds as the current ranges of stock lenses and semi-finished lenses.
It is an object of the present invention to provide laminate back wafers which can be combined with a variety of laminate front wafer designs and powers to produce lenses having through power accuracy comparable to those obtained in the manufacture of lenses from semi-finished lenses by the grinding and polishing route.